When British ships are in trouble and want to send a coded distress signal, for example if boarded by pirates, they will hang the union flag incorrectly. Any British vessel seeing another ship flying the flag upside down will know that there is a problem, but the crew cannot openly send a Mayday signal.

Was President Trump trying to acknowledge the distress of the United Kingdom after the terrorist bomb in Manchester this week - or do his team just not know how to display the British flag?

Donald Trump shares this confusion over the British flag with many British people, who also don't know how to display their own flag correctly. According to a poll by YouGov published earlier this year, 55% of Brits could not choose which flag is displayed correctly when presented with different options.

The YouGov research also shows that most people don't know what to call their flag. 49% believed it is the 'Union Jack' and 27% said the 'Union flag', but just 16% gave the right answer - which is that both are acceptable names for the flag.

It might seem trivial to many that the flag is displayed correctly, the British flag is often displayed upside down because many people assume the design is symmetrical, but the Brexit debate means that British people may soon need to have a more intimate understanding of their flag.

The current design has existed since 1801. It combines the crosses of St Andrew, St Patrick, and St George representing England, Scotland, and Northern Ireland. Wales was united with England in the 13th century and therefore the flag designers in later years didn't feel there was any need to add the Welsh dragon.

Could Brexit change all this? Nobody knows how the negotiations will go and what the eventual effect will be on the United Kingdom, but if the coming decades see an independent Scotland and united Ireland then England and Wales will need to design their own new flag. All the Commonwealth nations that use the union flag as a component of their flag design will also need to change.

The present union flag will be around for a few years to come. It would be nice if world leaders knew how to display it correctly even if most British people don't.